The present invention relates to method and apparatus for wrapping goods in a sheet of an elastic and stretchable film of a plastics material such as polyvinyl chloride or ethylene vinyl acetate copolymer.
Method and apparatus of the class specified above are disclosed in U.S. Pat. No. 3,662,513 issued May 16, 1972 to Ermanno Fabbri, Italy. According to the U.S. Patent referred to, a sheet of film intended to form a packaging envelope and severed from a continuous web is set up in stretched condition under a folding matrix plate having therein a passage opening corresponding to goods to be wrapped or packaged. The goods are then moved vertically upwardly through the passage opening in the folding matrix plate while at least two opposite edges of the sheet are held fast so that the sheet is upwardly stretched in the central portion thereof to form a bubble or bag-like envelope and is drawn over the goods. The edges of the stretched sheet are then folded under the goods and pressed against the under surface thereof to complete a package of the goods.
The method and apparatus disclosed in the U.S. Patent have advantageously solved the problem that stretchable film materials usable with stretch packaging were very difficult to handle because of their inherent properties, i.e., softness, ductility, deformability, adhesiveness and tendency of forming folds. The prior art method and apparatus, however, had a difficulty in that the goods were urged against the sheet to upwardly stretch the sheet for thereby forming the bubble or bag-like wrapper. There were many cases in which goods to be wrapped had irregular shapes and had projections extending from the general contour of the goods. In addition, when goods to be wrapped were foods, they were usually placed in a tray of a molded plastics material. When such a tray with irregular-shaped goods thereon was wrapped in a sheet of such a stretchable film material, the projections on the goods and sharp edges of the tray were brought into frictional engagement with the sheet during the stretching thereof with the result that the sheet was unevenly stretched. A part of the sheet between projections on the goods was stretched more or less than other parts of the sheet extending between projections on the goods and the edges of the tray depending upon the difference in friction between the projections on the goods and the sheet and between the tray edges and the sheet. In some cases, stretched sheets were broken at the point of engagement with such projections and tray edges. Thus, a stretchable film to be used with the prior art had to be selected on the consideration of the slidability of the film with respect to particular shapes of goods to be wrapped and the material from which a tray to be associated with the goods was made. On the other hand, such a film material was also required to have an adhesiveness so as to facilitate convenient adhesive sealing of edge portions of the sheet against the bottom surface of the tray. The slidability and adhesiveness are the opposite properties and, thus, it was impossible to obtain a stretchable film material which would meet with all of the requirements. Moreover, the slidability and adhesiveness are greatly influenced by the ambient temperature. The prior art, therefore, uneconomically needed a number of different kinds of film materials for different goods, different trays and different seasons.
The present invention aims to provide method and apparatus for wrapping goods in a sheet of a stretchable plastics film material by which frictional engagement between a sheet and a tray as well as goods thereon is avoided during a sheet stretching step to thereby eliminate the prior art difficulty discussed.
According to one feature of the present invention, there is provided a method of wrapping goods in an elastic and stretchable plastics film paterial, comprising feeding a sheet of the plastics film material to a predetermined position above goods to be wrapped and beneath a resiliently deformable bearing means, subjecting said sheet to a differential pressure to form an upwardly stretched bubble in the central zone of said sheet while portions of said sheet adjacent at least two opposite edges thereof are held fast until the top of the bubble is engaged by said bearing means, causing relative movement of said goods with respect to the bubbled sheet to introduce said goods into the bubble until the top of said goods is urged against the portion of said sheet engaged by said bearing means, and tucking the edges of said sheet against the side of said goods remote from the bubble to secure said edges together. As is conventional, the term "goods" as used herein means one or more articles (either on a tray or separate from a tray).
According to another feature of the invention, there is provided an apparatus for wrapping goods in an elastic and stretchable plastics film material, comprising a wrapping station, resiliently deformable bearing means at said wrapping station, means beneath said bearing means for supporting goods to be wrapped, means for feeding a sheet of the plastics film material to a position beneath said bearing means and above goods supported on said goods supporting means, means for producing differential pressure across the thickness of said sheet until the top of the bubble is engaged by said bearing means, means for holding fast portions of said sheet adjacent at least two opposite edges thereof while said sheet is subjected to the differential pressure, means for causing relative movement of said goods supporting means and goods thereon with respect to the bubbled sheet to introduce said goods into the bubble until the top of said goods is urged against the portion of the bubbled sheet engaged by said bearing means, and means for tucking the edges of said sheet against the side of said goods remote from the bubble to secure said edges together.
Preferably, the differential pressure may be produced by applying a gaseous pressure, such as pressurized air, to the under surface of the sheet and/or applying suction to the upper surface of the sheet. The gaseous pressure may be either produced in the apparatus or obtained from an external pressure source. In the case where goods to be wrapped are fresh foods, the sheet may advantageously be stretched and bubbled by a pressurized gaseous medium which would be effective to keep the foods sound, such as N.sub.2, O.sub.2 or CO.sub.2.
The deformable bearing means may be a pad of a resiliently deformable material. Alternatively, the bearing means may comprise a deformable bag of a fluid impermeable flexible sheet material and containing a fluid. The bearing means is depressed upwardly and resiliently bears the top of a bubble when the top of the bubble is urged against the bearing means by goods to be wrapped. The upwardly depressed portion of the bearing means is in intimate frictional engagement with the upper part of the bubbled sheet, so that the marginal portions of the sheet are not liable to be upwardly pulled by the resilient shrinkage of the stretched upper part of the bubble when the marginal portions are tucked. In an embodiment in which suction is applied to the sheet to form a bubble, the bearing means may advantageously comprise a pad of a gas permeable cellular material having open cells therein, such as a foamed plastics material, so that air above the sheet is evacuated through the open cells in the pad.
Intermittently driven endless belts may be used to draw a length of a web of a stretchable film material from a roll of the web and feed the drawn length of web to the wrapping station at which a sheet of the film may be severed from the continuous web. Alternatively, a fluid pressure actuation piston-cylinder assembly may be employed to feed a length of the web to the wrapping station.
The tucking means employed in the apparatus of the invention may preferably comprise four tucking plates diposed to form a rectangle extending around lower part of a bubble in which goods to be wrapped are introduced. The tucking plates of each pair may be disposed in substantially horizontally aligned opposite relationship with each other and movable toward each other to tuck the marginal portions of the bubbled sheet against the bottom surface of the goods.
The adhesiveness inherent to stretchable plastics film material is generally sufficient to retain the tucked marginal portions of a sheet of the film on the bottom surface of a tray for a while. However, the tucked edge portions may positively be secured together preferably by means of heat sealing if a produced package of goods is intended for storage for a long time.
According to the method and apparatus of the present invention, goods to be wrapped are not brought into frictional engagement with the central portion of a sheet of a stretchable film material during a stretching step with a resultant advantage that the central portion of the sheet can be uniformly stretched throughout the central portion. This advantageously eliminates the prior art problem that a particular kind of a film material had to be used with a particular shape of goods, with a tray of a particular material and at a particular ambient temperature.